The present invention relates to an electron tube provided with a photocathode for emitting electrons in response to incident light through the photoelectric conversion, and an electron multiplying section for multiplying the emitted electron flow through the emission of secondary electrons.
A photomultiplier tube, which is one of electron tubes, is used widely for various measurements in such fields as nuclear high-energy physics and nuclear medicine.
FIGS. 1(a) and 1(b) show an example of a conventional photomultiplier tube, including a top view and a cross-sectional view respectively. This photomultiplier tube includes a circular faceplate 11 for receiving incident light; a photocathode 20 formed on the inner surface of the faceplate 11 and held at zero potential; and an electron multiplying section 24 including a plurality of stages of dynodes 24a-24n. The first stage through mth stage dynodes 24a-24m are arranged in continuous stages. An anode 26 is positioned beneath the mth stage dynode 24m. The final stage dynode 24n is disposed directly beneath the anode 26. The first stage dynode 24a has a positive potential in relation to the photocathode 20. Electrons emitted from the photocathode 20 impinge on the first stage dynode 24a. The dynodes 24a-24m are formed with a plurality of electron multiplying apertures arranged in a matrix pattern. A focusing electrode 21 is formed with an electron focusing section 21a, disposed between the photocathode 20 and the electron multiplying section 24, and maintain at the same potential as that of the photocathode 20. Accordingly, photoelectrons emitted from the photocathode 20 are converged by the electron focusing section 21a and subsequently emitted onto a prescribed area of the first stage dynode 24a. 
In this type of conventional photomultiplier tube, however, the sensitivity of the photocathode deteriorates after a long period of use. As a result, the output from the photomultiplier tube in response to incident, light declines. This type of problem is particularly prevalent in photomultiplier tubes using a semiconductor photocathode, such as gallium arsenic (GaAs).
In view of the foregoing, it is an object of the present invention to provide an electron tube that has a photocathode and an electron multiplying section and is capable of preventing the deterioration of the photocathode to maintain a stable output over a long period of use.
To achieve these objectives, the inventors investigated the causes for deterioration in the photocathode. They discovered that positive ions were generated by the collision of electrons with a cesium (Cs) cloud formed around. the electron impinging section nearest the photocathode. The positive electrons were accelerated toward the photocathode due to the electric, field present at the site of their generation, resulting in ion feedback colliding with the photocathode. The inventors discovered that this collision caused the photocathode to deteriorate.
It should be noted that the potential of electrodes is defined by the positive or negative potential differential between electrodes rather than the absolute value of potential. In other words, when electrode A has a positive potential in relation to electrode B, the potential of the electrode A is higher than that of the electrode B.
According to one aspect of the present invention, an electron tube includes a photocathode that emits electrons in response to incident light through the photoelectric conversion; an electron multiplying section that multiplies electrons emitted from the photocathode, the electron multiplying section including an electron impinging section positioned nearest the photocathode, wherein the electrons emitted from the photocathode impinge on the electron impinging section; an ion confining electrode provided between the photocathode and the electron multiplying section for confining positive ions generated in the electron multiplying section; and an ion trap electrode provided between the ion confining electrode and the electron impinging section for capturing the positive ions confined by the ion confining electrode. The potential of the electron impinging section is set higher than the potential of the ion confining electrode. The potential of the ion trap electrode is set equal to or greater than the potential of the photocathode and lower than the potential of the electron impinging section.
In this type of electron tube, external light striking on the photocathode is converted to photoelectrons, which are accelerated toward the ion confining electrode having a positive potential in relation to the photocathode. After passing through apertures formed in the ion confining electrode and the ion trap electrode, the photoelectrons strike the electron impinging section of the electron multiplying section. At this time, positive ions are generated near the electron impinging section.
With the electrode configuration of the present invention, the generated positive ions are accelerated toward the photocathode. However, since the ion confining electrode has a positive potential in relation to the electron impinging section, the positive ions cannot pass through the apertures in the ion confining electrode to reach the photocathode. Ultimately, the positive ions are captured by the ion trap electrode which is set at a lower potential than the potential of the ion confining electrode and the electron impinging section, or by the electron impinging section itself, thereby preventing deterioration of the photocathode.
The potential of the ion confining electrode is set higher, within a range in which photoelectron converging from the photocathode to the electron multiplying section is not lost, than that of the electron impinging section at which positive ions are generated. Accordingly, ion feedback and deterioration of the photocathode caused thereby can be effectively suppressed without reducing the photoelectron capturing efficiency.
According to another aspect of the present invention, the electron multiplying section includes a plurality of stages of dynodes for capturing and orderly multiplying the electrons emitted from the photocathode. Here, the first stage dynode functions as the electron impinging section. According to another aspect of the present invention, the electron multiplying section is a microchannel plate having a plate structure formed of a plurality of bundled glass pipes. In this case, the microchannel plate is disposed so that one surface opposes the photocathode to serve as the electron impinging section. The electrons multiplied by the electron multiplying section are output from the anode electrode in the form of an electric current.
The present invention is particularly effective for electron tubes including a photocathode formed from a semiconductor photoelectric conversion material, such as gallium arsenic. However, deterioration of the photocathode caused by ion feedback commonly occurs in electron tubes using other types of photocathodes and affects the life span of such photocathode. Accordingly, the electrode configuration and potential settings of each electrode of the present invention are also effective for electron tubes using photocathodes formed from materials other than semiconductor materials.
According to another aspect of the present invention, the electron tube further includes a focusing electrode disposed between the photocathode and the ion confining electrode for converging the electrons.
According to still another aspect of the present invention, the ion confining electrode and the ion trap electrode can be formed with a row of a plurality of slits for allowing photoelectrons to pass therethrough. Alternatively, the ion confining electrode and the ion trap electrode can be formed with a plurality of channels arranged in a matrix pattern to allow photoelectrons to pass therethrough.